This is a discussion on Aftermarket Gauge FAQ: Read this before you buy within the Interior Mods forums, part of the Tech & Modifying & General Repairs category; Gauge FAQ
Who are the major gauge manufacturers?
Omori USA Home
Prosport Gauges
Auto Meter
日本精機のホームページ (Defi and STi Genome ...

Please note that there are more gauge manufacturers and this listing contains only the most talked about gauges on ClubWRX/NASIOC.
Who makes the best gauges? There is no best gauge manufacturer. Times change, but currently and historically the most popular manufacturers are Autometer, Defi, STi Genome, Prosport, Omori, and MadDad.
What gauges should I have? That is the question of the century with no real answer. We will discuss the following theories though:

On an unmodified vehicle no gauges are needed. This is a fairly safe statement. If more gauges were truly needed, they would be supplied by the factory as original equipment. If one would like to argue this point, you are more than welcome to argue this with SOA or Fuji Heavy Industries to your heart痴 content.

For a turbocharged car, the must have gauge is the turbo boost. For a singular mounting, the OEM position of on the steering column is considered the easiest and most discrete location. Why this gauge? Even on an unmodified vehicle, there have been discoveries by owners who have installed this gauge that they are not seeing stock target boost levels. The only way to see if your car is hitting target boost is through this gauge. While there is no danger in less boost, most would want at least the OEM target levels thank you very much.

What three gauges? This is generally the #1 gauge question due to the popular A pillar gauge pod or the factory clock gauge pod that hold three gauges. The correct answer is there is no correct answer, but the most popularly used gauges are: Boost, Exhaust Gas Temperature (EGT), and Oil Pressure.

Why these three?

Boost: 100% of engine management systems including the stock one have target boost levels. The only way of ensuring you meet and not exceed these levels is with this gauge. It is considered by most to be the #1 gauge for function and ease of installation. The boost is read off of a tap in the bypass valve line and no other location.
EGT: Monitors your exhaust temperature. This gives you a relative indication of A/F ratio and knock/detonation threshold. When used correctly, this can give you can indication when its time to back off of aggressive driving. Mind you, this gauge is not to be considered an A/F or detonation monitoring device as there is no direct means of conversion. EGT should be measured via tapping into the #4 runner of the exhaust manifold and no other location.

Oil Pressure: The main consideration for oil in a motor is a loss of oil, of which this gauge gives you a relative indication. Yes, oil can be too hot or cold, but neither condition will lock up your motor; loss of oil can. Mind you, this gauge is not to be a replacement for checking your oil level as pressure does not equate to oil level on a one for one basis. Oil pressure may be read from either a gallery plug or a sandwich adapter.

What are the other gauges available or to consider?

Clock: Yes, a clock. Since the OEM clock is often deleted due a dash housing that replaces it, a clock gauge is an option for replacement.
Coolant Temperature: Yes, there is an OEM idiot gauge for this function, but the OEM unit is designed to be heavily dampened which leads to sluggish movement and slow response. They also only supply a general indication. This means that when you notice you are hot, you have no idea exactly how hot your car is or how long it took to get to that state. Aftermarket units tell you the exact temperature and many gauges feature warning lights or buzzers set by the end user as a true warning. Keep in mind that temperature probe placement plays a role as a probe placed after the thermostat will only see correct coolant temperatures after the approximate 15 minutes it takes for the thermostat to open and coolant to fully flow within the system.

Oil Temperature: Why is this important? Oil has a sweet spot. Oil works best over a narrow temperature range of 220ｺ to 260ｺ F. Above 285ｺ F motor oil oxidizes and begins to turn to varnish. Below 220ｺ F, oil is too cool to boil off trapped/suspended water and excess fuel. Another rule of thumb is that oil temps should be about 50 to 60ｺ F hotter than coolant temperatures. This is why the generally preferred method of cooling oil is to employ an oil to coolant heat exchanger because it has the added benefit of warming the oil when cold.
Fuel Pressure: Shows how much fuel pressure is headed towards your stock or aftermarket fuel rails. This is critical for people who have an adjustable or modified fuel pressure regulator.

Voltage: Shows you the voltage your electrical system is producing.
Air/Fuel Ratio: We have two types here that need discussion.

A true A/F ratio gauge is used with a dedicated wideband oxygen sensor to provide A/F ratio data for tuning purposes. This gauge provides tuning support via quantitative A/F data. These are generally sold as sensor/gauge packages in the neighborhood of $300. Wideband systems are sometimes called "UEGO" for Universal Exhaust Gas Oxygen. The purpose behind this unit is to accurately determine the ratio of air vs. fuel. The reason this is important is for tuning purposes as a leaner mixture, to a point, creates more power than a richer mixture. Various tuners use various proportions though as this is not a how to guide on tuning. Its basic purpose is to allow the tuner to ensure the mixture is neither too rich nor too lean as both cause issues with EGT and/or detonation.

The other type is a unit designed to be wired into the factory oxygen sensor. The factory oxygen sensor is not a wideband unit that can be used for tuning. An A/F gauge that is wired into the factory sensor provides pretty lights that are 100% meaningless to air, fuel, or how much change is in your ashtray. So, if you have an empty slot in your three gauge unit that you would like filled, go for the clock gauge as it actually has a function and purpose. Seriously, this is a useless gauge.

What type of gauge should I get, mechanical or electrical?

Mechanical Gauges:

Mechanical gauges do not require power to operate.
They can use power for lighting purposes though.
They make direct physical contact with the item they are reading.
Mechanical gauges can have noise to them. Some might have a slight rattle as there are moveable gears, pins, or pivot points inside which may cause noise.
Can have a jumpy or have an erratic sweep.
Generally less expensive.

Electrical Gauges:

No large connectors and tubing coming out the back.
Can be mounted in more unusual positions without connections showing.
Easier to install in tight areas.
Easier to install a great distance from the item being measured.
Stops fluids from entering the passenger compartment.
The pointer sweep is more smooth and seamless.
Generally more expensive.

Which is more accurate though? For 99% of users either will work just fine. In theory, there is a slight advantage towards mechanical gauges, but recent electronic advances have narrowed the accuracy to the point where it is really a moot issue.

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